Chapter 38 PLANT REPRODUCTION

AND BIOTECHNOLOGY

The life cycle of angiosperms is characterized by an alternation f generations, in which the haploid (n) and diploid (2n) generations take turns producing each other.

SEXUAL REPRODUCTION

Plants have a characteristic life cycle called alternation of generations.

·  Sporophyte generation is diploid, 2n, and produces haploid spores through meiosis.

·  Gametophyte generation is haploid, n, and produces haploid gametes through mitosis.

·  Fertilization restores the diploid stage; meiosis restores the haploid stage.

In flowering plants the diploid sporophyte generation is larger and nutritionally independent.

·  The sporophyte produces spores by meiosis.

The haploid gametophyte generation, which is located in the flower, is microscopic in size and nutritionally dependent of the sporophyte.

·  The gametophyte gives rise to gametes by mitosis.

The pollen grain is the male gametophyte of angiosperms.

Pollination is the transfer of pollen from the male structures to the female structures in the same or different flower.

Fertilization is a different process and takes place inside the ovary of the flower as does the development of the seed.

The ovary becomes the fruit, another unique structure of the angiosperm.

The flower

The flower is a modified branch apex, and is involved in sexual reproduction. It is a unique structure of the angiosperms.

Flowers are a collection of organs designed to produce gametes, attract gametes, and develop seeds.

Most plants reproduce sexually.

·  Meiosis: reduction in the number of chromosomes.

·  Gametogenesis: Formation of sperm and eggs.

·  Fertilization: fusion of sperm and egg.

Variations in flower structure.

Floral whorls:

·  Complete: sepals, petals, stamens and carpels

·  Incomplete flowers lack one or several floral whorls.

Sexual parts of the flower:

·  Perfect: bisexual, both stamens and carpels present.

·  Imperfect: unisexual, the flower has only stamens (staminate) or carpels (carpelate).

Sex of the plants:

·  Monoecious: the plant has both staminate and carpellate flowers; flowers imperfect.

·  Dioecious: staminate and carpellate flowers are found on separate plants; flowers imperfect.

·  Hermaphroditic: plant with flowers perfect.

·  "Oikos" = house

Arrangement of floral parts: spiral or whorled.

Many plants are capable of outcrossing, that is, they exchange gametes with other plants of the same species.

Many plants are also capable of self-fertilization.

Reproductive and accessory organs are normally arranged in whorls or circles of structures:

·  Sepals, petals stamens, and carpels.

The whorls of organs sit on an enlarged branch end called the receptacle.

The sepals form the calyx and protect the flower bud.

The petals form the corolla and attract animals to assist in pollination. Petals may or may not be present.

The stamens are the male reproductive organs.

·  They consist of a filament and an anther.

·  Pollen grains form in the anthers.

·  Each pollen grain contains two cells; one produces two sperm nuclei, and the other produces a pollen tube to transfer the sperm nuclei to the ovule.

·  A pollen grain represents a male gametophyte.

The carpels are the female reproductive structures.

·  A flower may have one or several carpels.

·  Carpels may be separate or fused.

·  Carpel usually has a style and stigma.

·  Ovary is another name for the lower portion of the carpel.

·  An ovary may be formed by various fused carpels.

·  Pistil is another name for the female reproductive structure.

·  A pistil may be formed by a single carpel or by several fused carpels.

·  The ovary contains one or several ovules.

·  The ovule produces contains the embryo sac.

·  The embryo sac produces two polar nuclei and one egg.

·  The egg and the polar nuclei are involved in the process of double fertilization.

Producing the male gametophyte

ANTHER → SPOROGENOUS TISSUE → MICROSPORE → MICROGAMETOPHYTE (POLLEN GRAIN) → GENERATIVE CELL → MALE NUCLEI

  1. The anther and the filament make the male reproductive organ of the plant.
  1. Inside the anther, the diploid cells of the sporogenous tissue divides meiotically to produce haploid microspores.
  1. Each diploid cell produces four haploid microspores.
  1. Each of the four cells forming called now microspores, divide mitotically to form pollen grain made of generative cell enclosed in a larger vegetative cell.
  1. A very resistant outer layer of sporopollenin and an inner layer of pectin surround this two-cell structure or pollen grain.

Producing the female gametophyte.

OVULE → MEGASPORE → MEGAGAMETOPHYTE (EMBRYO SAC) → EGG

  1. The ovule consists of an inner tissue called the nucellus and one or two protective layers called integuments.
  1. The integuments form one small opening at one end of the ovule, the micropyle.
  1. The one diploid cell in the nucellus produces four haploid cells or megaspores through meiosis.
  1. Three nuclei degenerate and one haploid nucleus remains.
  1. The functional megaspore enlarges at the expense of the other cells of the nucellus.
  1. This nucleus inside the cell divides mitotically twice, forming first four nuclei and then eight nuclei.
  1. The eight nuclei separate and form six small cells and one large cell, the megagametophyte or female gametophyte.
  1. Three cells migrate towards the micropylar end, one cell becomes the egg and the two others are called synergids eventually degenerate.
  1. Three other cells migrate to the end opposite to the micropyle.
  1. Two nuclei remain the in the center of the large cell. These two nuclei are called the polar nuclei.
  1. At this point this structure is called the female gametophyte or megagametophyte or embryo sac.

Self-incompatibility

Some plants can self-fertilize if pollen from the same plant fall on the stigma.

Other plants cannot self-fertilize: these plants are self-incompatible.

There are several mechanisms involved in the compatibility or not of the pollen and the stigma.

The S locus in the cabbage family:

  1. The "S locus" consists in reality of three loci.
  2. There are multiple alleles of these genes, up to 50.
  3. The proteins coded by these loci are located one in the membrane of the stigma cells, another in the cell wall of the stigma cells, and the third is secreted by mature pollen grains.
  4. If the proteins secreted by the pollen are the same as one or both of the proteins in the cell membrane and wall of the stigma, the pollen grain does not form a pollen tube. Similarity of alleles means that they are probable from the same plant.
  5. If the proteins secreted by the pollen tube are different from both of those in the cell wall and the cell membrane of the stigma, the pollen tube forms. The pollen comes from a different plant.

Fertilization is the fusion of gametes.

·  It restores the diploid condition in the zygote.

The male gametophyte produces a long tube that grows through the stigma, the style and enters the micropyle of the female gametophyte (syn. embryo sac).

The generative cell of the pollen divides to form two sperm nuclei that move into the pollen tube.

Double fertilization is a unique phenomenon that occurs in angiosperms only.

·  Egg and one sperm form the zygote, 2n.

·  The two polar nuclei and the second sperm form the endosperm, 3n.

The endosperm stores food for the developing embryo.

The ovule will develop into a seed and the ovary will develop into a fruit.

Endosperm development

After double fertilization, the triploid nucleus of ovule's central cell divides mitotically forming a multinucleate large cell.

The liquid mass of the endosperm becomes multicellular when cytokinesis forms plasma membranes and divides the endosperm into many cells.

Then these cells develop cell wall around them and the endosperm becomes solid.

Embryogenesis

Embryonic development follows a pattern:

The zygote divides into two cells; the upper cell develops into the embryo proper, and basal cell develops into a column of cells that brings in nutrients and gibberellins to the developing embryo.

The suspensor is short lived and undergoes a process of programmed cell death or apoptosis.

Proembryo ® globular embryo ® heart-shaped embryo ® ® mature embryo.

The mature embryo

A mature embryo consists of a radicle, hypocotyl, one or two cotyledons and the plumule.

Radicle: embryonic root. Cotyledons: embryonic or seed leaves.

Hypocotyl: embryonic or seed stem. Plumule: shoot apex or meristem.

In some species, the cotyledons absorb the food from the endosperm and become the storage structure, e. g. beans, peanuts.

In other species, the food remains stored in the endosperm and their cotyledons are very thin.

Seeds of monocots have a single cotyledon.

·  The grass family has a specialized cotyledon called the scutellum.

·  The scutellum is thin and in contact with the cotyledon.

·  During germination, the scutellum absorbs food from the endosperm and transfers it to the developing embryo.

·  The embryonic root is covered by protective sheath called the coleorhiza, and the shoot apex by a sheath called the coleoptile.

The fruit

Seeds are enclosed in fruits.

Fruits are ripened ovaries and may or may not include other parts of the flower.

The fruit protects the seed and aids in seed dispersal.

The ovary wall or pericarp thickens during maturation.

Fruits may be fleshy like pears and oranges, or dry like wheat and string beans.

As the fruit ripens, enzymes digest the cell walls and organic acids and starch are converted to sugars, which may reach a concentration of 20% in some cases.

Dry fruits have a dry pericarp and are usually confused with seeds, e. g. wheat, corn and acorns.

Seed dormancy

Seed dormancy is characteristic of plants that live in seasonal climates.

There is not one single mechanism involved in seed dormancy.

In some plants, dormant seeds have high concentration of abscisic acid (ABA), which inhibits germination.

When the dormant seeds are exposed to water, the abscisic acid washes out and germination starts.

Some seeds have a very thick and resistant coat that does not allow water or oxygen to reach the embryo. These seeds must be scarified by abrasion, fire or passage through the digestive track of an animal.

Other seeds must undergo a period of cool or freezing weather before germination.

Seed germination

During germination, glucose breakdown may be entirely anaerobic.

·  Increase in water uptake, oxygen consumption, and protein production.

·  Cells enlarge and the embryo burst through the seed coat.

·  Radicle emerges and begins to penetrate into the soil.

·  Hypocotyl elongates.

·  The process is controlled by mRNA stored in the seed.

There are at least three mechanisms used by the germinating seed. See fig. 38.14, page 794.

ASEXUAL REPRODUCTION

Asexual reproduction is also called vegetative reproduction.

Offspring are formed without the fusion of gametes.

Offspring are genetically similar to the parent plant.

Stems, leaves and roots may be adapted to asexual reproduction.

Apomixis is the formation of seeds without fertilization, akin to parthenogenesis is animals.

·  The new generation can come from an unfertilized ovule or from a vegetative cell.

Modified stems may give rise to independent plants in time by fragmentation.

Modified leaves can produce plantlets that break off and give rise to new plants.

Roots can produce aerial shoots that become adult plants. These new plants are clones of the original parent.

Sexual and asexual reproduction are complementary in the life of plants.

Asexual reproduction enables successful clones to spread; sexual reproduction generates the genetic variation that makes evolutionary adaptation possible.

Grafting allows combining the best characteristics of two plants.

·  The plant that provides the root system is called the stock.

·  The twig that is grafted is called the scion.

Vegetative reproduction of plants is common in horticulture.

Desirable varieties can be cloned from single cells taken from the parent plant.

Shoot or stem cuttings can be used to form undifferentiated masses of cells called a callus that eventually will form roots and a shoot.

PLANT BIOTECHNOLOGY

Agriculture began between 11,000 and 10,000 years.

From that time on, humans have been selecting plants for their desired characteristics by means of artificial selection.

Interspecific hybridization is common in plants, and has been used by breeders to introduce new genes into crops.

Modern plant biotechnology uses techniques of genetic engineering to introduce genes of unrelated species.

These organisms that have genetically engineered to express a foreign gene from another species are called transgenic.

Genetically modified plants have the potential of increasing the quality and quantity of food.

There are many concerns about the unknown risks of releasing genetically modified organisms into the environment.